On Mon, 25 Nov 2013 09:59:05 +0000, Chris J Dixon wrote:

The simplest, and crudest, solution is the attack the screw head with a
Dremel cutting disk, and attempt to cut a sharp enough edge to engage a
screwdriver. This will inevitably cause damage to the surrounding
plastic.

Since it's clearly outside warranty and you'll not be worried about any
future manufacturer support, the only real problem would be one of
cosmetic damage. I'd have thought a well-used garden shredder would
already be a little, umm, "pre-loved" in appearance anyway.

I'd just do it.

But have you tried drifting it with a small screwdriver or chisel or
similar, first? I can't imagine it's particularly tight. Might just come.